1. Field of the Invention
The present invention relates to a cold rolled steel sheet having excellent cold rolling efficiency together with excellent press formability by means of continuous annealing, and to a method for producing the same.
2. Description of the Prior Art
Since a cold rolled steel sheet having a good press formability has been heretofore manufactured with the chief aim directed to the mechanical properties of the steel sheet, the chemical composition and processing conditions thereof have been substantially decided upon. Quite recently, the thickness of a hot rolled steel strip has increased in order to save the energy required and to attain high productivity for hot strip rolling. Thus, the development of such a cold rolled steel sheet having a sufficient rupture strength in the cold rolling, and lower energy consumption required for cold rolling together with good press formability, and a method for producing the same, are now in strong demand.
As a method for producing a deep drawing steel sheet using continuous annealing, it has been known to coil a hot rolled steel sheet at high temperature in a hot strip mill. For instance, there is a method of coiling a steel containing C.ltoreq.0.06% by weight at 630.degree. C. or higher (Japan Examined Patent Application No. 1969/74); and another method for coiling a steel containing 0.01.about.0.10% Mn by weight, less than 0.003% S by weight, less than 0.005% P by weight, less than 0.006% N by weight, and 0.01.about.0.06% Al by weight at 650.degree. C. or higher (Japan Laid-Open Patent Applicatin No. 35726/81) has also been proposed.
The former relates to an improvement in the deep drawability of the steel sheet by a method which comprises coiling the hot rolled steel strip at a high temperature in order to coarsen the carbide of the hot rolled steel strip, and the P and N contents of the steel are on a level with common Al-killed steel. The latter is directed to an improvement in the deep drawability of the steel sheet by a method which comprises extremely lowering the Mn content and S content as well as the P content in addition to the high coiling temperature, but the N content is on a level with common Al-killed steel.
However, in both of the above-mentioned methods, high temperature coiling is performed during the hot rolling step. Therefore, when the steel coil is cooled, the cooling is non-uniform throughout. As a result, the uniformity of mechanical properties in the longitudinal direction as well as the width direction is lowered. Particularly, the quality of the top and bottom ends of the coil is so extremely deteriorated as to seriously reduce the yield of the steel product. In addition, a thick scale is produced by the high temperature coiling, hence there is the disadvantage that descaling efficiency of the hot rolled steel strip is low.
Furthermore, the deep drawable steel sheet containing B is disclosed in U.S. Pat. No. 4,410,372; and Japan published unexamined patent application Nos. 145,123/80, 94,446/80 and 135,616/79.